1. Field of the Invention
The present invention relates to a pressure-welded semiconductor device using a semiconductor element.
2. Description of the Related Art
A pressure-welded semiconductor device has a structure where at least one electrode disposed on each of two main surfaces of a semiconductor element (hereinafter abbreviated as “element-side electrodes”), such as an insulating gate bipolar transistor (hereinafter abbreviated as an “IGBT”) or a MOS field-effect transistor (hereinafter abbreviated as a “MOSFET”), is pressurized by casing-side electrodes (hereinafter abbreviated as “casing-side electrodes”) that contact the element-side electrodes. The reliability of this pressure-welded semiconductor device is high because solder is not used to connect the element-side electrodes and the casing-side electrodes.
The real contact area in the contact surfaces between the element-side electrodes and the casing-side electrodes is controlled by a pressurizing force. When the real contact area is small, electrical resistance between the element-side electrodes and the casing-side electrodes becomes large and diffusion of heat generated in the semiconductor element becomes difficult. In this case, the pressure-welded semiconductor device becomes unable to exhibit sufficient performance as a pressure-welded semiconductor device and reliability drops. For this reason, it is preferable for the pressurizing force to be as large as possible in order to enlarge the real contact area.
However, when the pressurizing force is increased, it is difficult to miniaturize and reduce the weight of the pressure-welded semiconductor device, and manufacturing costs also become high, because the configuration of pressurized portions for pressurization becomes large.
Japanese Patent Application Laid-Open Publication (JP-A) No. 2000-68297 proposes disposing a conductive nonwoven fabric such as Ni at the pressure-welded portions between the element-side electrodes (intermediate electrode plates) and the casing-side electrodes (common electrode plates). In this case, in comparison with the above-described conventional pressure-welded semiconductor, electrical resistance can be kept low and heat can be efficiently diffused, even with a relatively low pressurizing force, because variations in the height of the contact surfaces can be sufficiently absorbed.
However, because a conductive nonwoven fabric is used, electricity and heat are transmitted only at contact points between the fibers, and the spaces between the fibers do not function as good conductors of electricity and heat. Thus, for example, in a case where the conductive nonwoven fabric comprises a metal such as Ni, the electrical resistance and heat conductivity of the Ni nonwoven fabric drops even more than the original values of bulk Ni.
Thus, when the technology described in JP-A No. 2000-68297 is compared with the case where a bulk conductive substance is disposed between the element-side electrodes and the casing-side electrodes by metal solder connection or the like, it is inferior in terms of the drop in electrical resistance and improving thermal conductivity.
Additionally, in the case where a pressure-welded semiconductor device is assembled using a conductive nonwoven fabric, the conductive nonwoven fabric must be precisely disposed between the element-side electrodes and the casing-side electrodes. When the nonwoven fabric is disposed between the element-side electrodes and the casing-side electrodes in a state where positional displacement has occurred, there is the potential for other wiring inside the pressure-welded semiconductor element and electrodes to make contact. In this case, the pressure-welded semiconductor device becomes unable to exhibit its original performance as a pressure-welded semiconductor element, and cases where problems such as the device electrically short-circuiting are possible. This problem occurs not only when the conductive nonwoven fabric is positionally displaced, but there is also the potential for this problem to occur due to a drop in fibers of a long fiber length resulting from deterioration over time.
Moreover, because it becomes necessary to prevent positional displacement itself of the conductive nonwoven fabric at the time of manufacturing the pressure-welded semiconductor device, the assembly steps of the pressure-welded semiconductor device become more complicated and manufacturing costs also increase.